# Picking block length in a block bootstrap

I am using the Mann-Kendall test to assess trends in a data time-series. I believe there is autocorrelation in my data and therefore need to use a block bootstrap to correct for it. I have plotted the autocorrelation to try to determine the block size to use in my block bootstrap. I have not found many resources on how to select this block? Comparing one example to my data I thought I should pick a length of $$1515$$ as at lag $$=15=15$$ the points stay within the autocorrelation intervals (blue lines) but another source said $$2-42-4$$ is usually a sufficient block length and that a block length of $$1/41/4$$ of the sample size ($$nn$$) can make the test insignificant. My data $$n=64n=64$$ so $$1515$$ is approaching $$1/41/4$$ of the sample size. How can I tell the best block size to pick from this plot (or another method?)